This invention relates to a method and an apparatus for coating a metallic wire with a similar or dissimilar metallic plating layer of the desired thickness by passing the wire through an electrolytic bath. More particularly, the invention provides a method and an apparatus for efficiently forming an electrolytic plating layer of compact metallic structure at a high current density.
When a thick electrolytic plating layer is to be formed on a wire, the wire usually must be passed through an electrolytic bath repeatedly a number of times to form the plating layer of predetermined thickness. If a high current density is given to the wire for the plating process, an increased amount of deposition will result per unit time, consequently reducing the number of repetitions of the electrolytic treatment.
However, with an increase in the current density, the plating layer formed around the wire becomes more rough-surfaced with marked irregularities, so that the repetition of the electrolytic treatment produces pronounced surface irregularities, failing to afford a compact plating layer.
U.S. Pat. No. 2,370,973 discloses an apparatus by which a wire coated with a compact plating layer can be produced even at a high current density. The apparatus comprises a series of electrolytic chambers provided in a straight pipe and partitioned by drawing dies. The chambers are connected to the anode of a d.c. power supply, and the drawing dies to the cathode thereof. While being passed through the pipe, the wire is electrolytically coated in one chamber, then drawn through a drawing die to smooth the plating layer, and further electrolytically coated over the smooth-surfaced plating layer in the next chamber. Thus every time the wire is electrolytically coated, the surface of the plating layer is smoothed with the drawing die.
With the apparatus described, the electrolytic plating layer formed at a high current density, even if rough-surfaced, is made smooth-surfaced by the drawing die prior to the subsequent electrolytic treatment, with the result that a wire coated with a compact plating layer can be produced at a high current density.
The apparatus nevertheless has the following drawbacks.
(1) Since the plating layer is smoothed by die drawing, the wire coated with a large amount of electrolytic deposition at a high current density is diametrically reduced every time it is smoothed. Accordingly the electrolytic step and the smoothing step must be repeated a large number of times for forming a plating layer of the desired thickness. (2) During the die-drawing smoothing step, the coated wire is subjected to tension due to the diametrical reduction. When treated for plating and smoothing repeatedly a substantial number of times, the coated wire will be loaded with severe tension, so that the number of repetitions of the above treatment is inherently limited. This imposes a limitation on the thickness of the plating layer which can be produced by a continuous process. When the apparatus is used for continuously producing a wire coated with a plating layer exceeding the limit in thickness, the apparatus requires an additional device for relieving the wire of the tension. This makes the apparatus large-sized and complex. (3) When worn, the dies must be replaced by a very cumbersome procedure, while the maintenance of the dies with the desired bore size needed leads to a reduced productivity.
Since these objections are encountered even with the apparatus of U.S. Pat. No. 2,370,973, usual electrolytic plating processes are carried out at a low current density which affords plating layers free of surface irregularities.